Jamming refers to intentional emissions of energy to interfere with operations of a communication device by decreasing its signal-to-noise ratio or saturating its receiver with noise or false information. Anti-jamming refers to techniques utilized to mitigate effects of jamming. Various anti-jamming techniques have been developed to provide protections against jamming. For example, techniques such as frequency excision and the like are commonly utilized to mitigate effects of jamming. A challenge in frequency excision is not only to identify which energy is a candidate for excision, but also to control the excision process in order to minimize effects on desired signals.
Traditional anti-jamming techniques generally rely on Fourier transformations or finite impulse response (FIR) filters. Typically, received signal samples are processed in units referred to as blocks or observation intervals, wherein each block is essentially a collection of samples to be processed in aggregate. It is noted, however, if the frequency of a jamming signal changes or if its amplitude pulses over the course of anti-jam processing, the effectiveness of the traditional Fourier transformations or FIR filters based anti-jam processing techniques may be compromised. For example, if the frequency of a jamming signal changes (e.g., the jamming signal may sweep across a frequency band), the bandwidth of the jamming signal may appear to be greater than it really is due to the limitations of the fixed block/observation interval. Similarly, if a jamming signal is pulsed on and off, an anti-jam processor may be forced to assume that a jamming signal is present during the entire observation interval. Therefore, traditional means of implementing frequency excisions may be ineffective against these types of agile jammers (e.g., pulsed and/or swept jammers) for a large range of relevant pulse and sweep rates. For example, even if anti-jam processing can be tuned for a short range of rates, the performance still suffers in some other aspect (e.g., lack high time resolution to address a scenario of many continuous wave jammers).
Wavelet transformation based anti-jam techniques described in: Method and Apparatus for Receiving a Geo-Location Signal, U.S. Pat. No. 7,778,367, which is herein incorporated by reference in its entirety, implement wavelet transformations instead of Fourier transformations or finite impulse response (FIR) filters based excisions. As described in U.S. Pat. No. 7,778,367, a particular jamming signal may be mitigated by performing spatial processing in time-frequency bins in a wavelet transformation. More specifically, once a unit of signal sample is obtained, a wavelet transformation may be applied to the unit of signal sample. It is noted that in contrast to traditional anti-jamming techniques which relied on a Fourier transformation, a wavelet transformation is generally accomplished according to a basis function. Where a Fourier transform generates a frequency domain rendition of a signal, a wavelet transformation generates a scale vs. time-interval rendition of a signal. In the wavelet domain, there is a known inverse relationship between the notion of scale and frequency. Herein we will use the term frequency with the understanding that the inverse relationship exists without loss of generality with respect to the wavelet transformation. As such, a particular transformation may, according to one example, define a particular number of frequencies over a particular period of time. Yet another example of a transformation may convert a time domain rendition of a signal into a frequency vs. time-interval rendition of a signal, wherein a particular frequency may span a different time-interval than a different frequency. With such control over the wavelet transformation, a jamming signal that occupies a particular band over a particular interval of time can be anticipated and mitigated.